1. Field
This invention relates generally to cables for transmitting data in the form of optical signals between electronic apparatuses, and in particular relates to cables for transmitting large amounts of data over long distances in temporary field operations of such a character that radio communications are impractical.
Specifically, the invention has application for cables that are used both (1) to collect seismic information from an array of many sensors spaced apart by only a few dozen feet or yards, and at the same time (2) to relay information between electronic apparatuses that service subgroupings of such sensors along the array, multiplexing the locally collected information for incorporation into an aggregate signal for transmission to a base station. The invention is not, however, limited to such application.
In such seismic exploration, problems of equipment weight, environmental interference with signal integrity, simplicity in use, field reliability, and ease of service under rugged field conditions when malfunctions do arise, can become paramount--even when extremely high-technology techniques have been built into the data-generation and data-interpretation operations.
The constraint of equipment weight in field operations favors use of optic fibers for transmission of as much data as possible, since optic fibers are extremely "fast" data-transmission links and are made of fine plastic or glass, and thus are very light in weight compared to electrical coaxial cable of comparable multiplex data-carrying capacity. The constraint of signal interference likewise favors use of optic fibers for transmission over the longest distances that may be involved, since such fibers are completely insensitive to electrical interference sources.
Effective use of optic-fiber cabling in rough field environments, however, requires solution of the other types of problems mentioned above.
For one thing, it is undesirable to make field connections between optical surfaces, on a wholesale basis, because optical interfaces are subject to contamination by fine dirt and dust.
For another, it is essential that when cabling is damaged--so that replacement/repair with some exposure of optical surfaces becomes necessary--the cabling be replaceable with a minimum of "fine work" or sophistication, and a maximum of reliability.
For yet another, because the cost of setting up such operations is extremely high, it is also desirable that installation be made as quick and as insensitive to ordinary human error as possible.
2. Prior Art
The closest prior device of which we are aware is disclosed in U.S. Pat. No. 3,792,284, issued to Oscar J. Kaelin. That device has a light emitter in one connector and a light detector in another, with an optical-fiber-containing cable interconnecting the two--so that light from the emitter is presented to the detector.
Kaelin's light detector is electrically connected directly to the pins of an electrical connector that is adapted to mate with a corresponding electrical connector of a receiving apparatus.
His detector and emitter are inserted into alignment sleeves that face the respective openings in his respective connector housings, and the optical-fiber cable ends are led into both housings and inserted so that the optical surfaces butt against the detector and emitter, respectively; the cable ends are then clamped in those respective positions.
The clamping serves the purpose of holding the cable ends juxtaposed to the detector and emitter, and it also serves the purpose of bearing any tensile force that may be applied between the cable and the connector.
It will be apparent that, barring cable damage, Kaelin's device resolves at least part of the problem of cable weight, and also the problem of making optical connections in the field, since the connectors of his cable are both electrical connectors.
If cabling were damaged and it became desirable to replace cable but use the same connectors, however, Kaelin's device could be less than ideal. Replacement requires having an optic-fiber cable with an optic fiber whose optical surface is flush with the end of the bulk cable and yet is of good optical quality and clean. Moreover, several inches of each end of the cable must be inserted into a connector precisely the correct distance so as to butt against the corresponding optical device to form a reliable optical interface for data transmission--but not to jam against the optical device and so misalign it, or damage either it or the fiber face.
Further, if Kaelin's clamp is tightened enough to reliably bear any tensile force that may arise between the cable and the connectors under field conditions, distortion of the optical-fiber cable end may arise sufficient to misalign the optical interface, as can be seen from Kaelin's drawing.
In addition, because Kaelin's detector is wired directly to the electrical connector, the high-impedance signal from the detector must pass through the connector interface into the receiving apparatus. Along that route of several inches--if Kaelin's device were used under the type of field conditions that are being considered here--the detector signal would be subject to electrical interference both from the environment and from the multiplexing electronics in the receiving apparatus.
Further, as Kaelin points out at column 2 lines 40 through 43 of his patent, necessarily with his device "[t]he connector pins 14 and 15 are arranged differently so as to identify the electrical input from the electrical output and to prevent improper connection to mating receptacles." This provision would of course defeat the consummation of human error in actually connecting his cable in the wrong orientation, but it does not defeat the preparatory stage of that same error--that is to say, it does not prevent the installer from laying out the cable over perhaps a mile of rugged terrain before discovering that it was lying backwards.
Thus, while the Kaelin device is undoubtedly useful for a variety of purposes, it would not be eminently suitable for the kinds of applications here under discussion. The present invention is aimed at providing a cable assembly that would obviate the above-described disadvantages of the Kaelin device for these applications, and that also would be more generally useable and useful with convenience, reliability, and high-fidelity data-transmission performance in a great variety of other applications.